Plasma processing systems are widely used to process substrates. Examples would be etching of silicon wafers in semiconductor manufacture and the deposition of layers in the manufacture of solar cells. The range of plasma applications is wide but includes plasma enhanced chemical vapour deposition, resist stripping operations and plasma etching.
There is a need in industry to deploy sensors in plasma processes to measure key process parameters as a function of position and time, in order to reduce process development time and increase process control, and for fault detection and resolution. These applications often require spatial mapping with the distribution of multiple sensors within the processing area.
In recent years a number of approaches to obtain position- and time-dependent data using in-situ and substantially real-time instrumentation and measurement have been proposed.
U.S. Pat. No. 5,746,513 discloses a temperature calibration substrate with a cavity located below the substrate surface of said and a thermocouple disposed in the cavity for measuring the temperature of the substrate. The cavity includes a cavity opening, an inner perimeter, and a length. Heat transfer means is disposed in the cavity means between the thermocouple and the inner perimeter of the cavity for transferring heat from the substrate to the thermocouple. The cavity is shaped to allow the thermocouple to lay in close proximity to the substrate, and the thermocouple is positioned substantially adjacent the inner perimeter of the cavity and traverses the length of the cavity means thereby enhancing heat transfer efficiency from the substrate to the thermocouple means.
U.S. Pat. No. 6,691,068 discloses a complete measurement system with distributed sensors on a workpiece. The system employs a sensor apparatus that includes an information processor, embedded executable commands for controlling the apparatus, and at least one sensor. The information processor and sensor are supported on the substrate. The sensor converts the measured operating characteristics into digital data, which is either stored in the sensor apparatus for later retrieval (i.e. when the work-piece is removed from the plasma chamber) or stored and transmitted wirelessly to an external receiver.
U.S. Pat. No. 6,830,650 discloses a wafer probe for measuring plasma and surface characteristics in plasma processing environment that utilizes integrated sensors on a wafer substrate. A microprocessor mounted on the substrate receives input signals from the integrated sensors to process, store, and transmit the data. A wireless communication transceiver receives the data from the microprocessor and transmits information outside of the plasma processing system to a computer that collects the data during plasma processing. There is also provided a self-contained power source that utilizes the plasma for power that is comprised of a topographically dependent charging device or a charging structure that utilizes stacked capacitors.
The use of wireless transmission (or storage and wireless transmission) to transmit the data from the sensors for subsequent analysis is not without problems.
In plasma processes the transmission of radio-frequency signals through the plasma is hindered by the fact that the plasma is a conductor, which shields the antenna. This can be overcome by ensuring the carrier frequency is higher than the electron plasma frequency, typically by using a carrier frequency in the 1-100 GHz band. Thus, it is possible to use a carrier in the microwave, infrared or optical portion of the spectrum (as suggested in U.S. Pat. No. 6,691,068), but this requires essential line of sight communication between the external receiving antenna and the antenna attached to the sensor. Alternatively, if lower frequencies are to be used, the data must be stored for transmission when the plasma is off, increasing the size and complexity of the plasma processing system.
The placing of a complete measurement system including sensors, multiplexing, digitizer, executable instructions and storage system on a work-piece means that the data from the sensors is digitised and stored on the work-piece. The work-pieces are often in hostile environments with RF and magnetic fields that increase the likelihood of noise contamination of the sensor data. Leads from the sensors to the microprocessor are particularly vulnerable, they can act as antenna and need complex shielding to minimise disturbance.
The local bias on the workpiece is often different at different locations and substantially different from the other parts of the tool. Therefore analog sensor data is modified and maintaining an electrically floating sensor is difficult.
Specifically, the requirement of the present art to digitise the sensor data means that an analog to digital converter (ADC) is located on the workpiece, and analog data needs to be routed across the workpiece to the ADC leading to issues with noise pickup.
It is also clear that the high power required to run an ADC, particularly where high speed and high resolution is required, is a limiting factor. If high-speed data is required the storage of data will consume larger amounts of space and power, which are limited inside the system.